District heating

Carsten Petersdorff 12.7.1 Concept

District heating is a collective heating system supplying several buildings. High-performance housing can also benefit from this shared approach. Given the very small heat demand of individual highperformance houses, the capital costs for the backup heating become very expensive. When one heating plant can serve many houses, the economics improve and there is a greater selection among types of systems. Examples include solar seasonal storage, waste incineration plants and gas engine heat pumps. An added advantage of such a large central facility is that they can be more easily adapted to future developments than exchanging the heating system in each individual building. The idea of such micro-heating networks derives from urban district heating, but design modifications are essential.

In the US as well as in some European cities, steam systems up to 175°C are common. In Europe, systems operating with hot water at temperatures of 90°C to 130°C are more common. Some systems even operate at temperatures below 90°C. Such lower temperature systems are well suited for local district heating, solar seasonal storage or heat pumps. Requirements for operation, distribution and onsite installation are more flexible and costs per unit of heat can be significantly lower than for high temperature systems. For all of these reasons, such systems are the best suited to serve high-performance housing.

A typical district heating system has two subsystems:

1 Heat generation: for district heating plants to produce hot water (or steam) may be relatively simple - for example, a single, large gas-fired boiler. To improve efficiency, however, multiple boilers are often built to cover base, intermediate and peak loads. A co-generation or renewable system can meet the base loads and a separate peak load system can provide standby capacity, as needed. This also increases system reliability.

2 Heat distribution: two concepts are common: two- and four-pipe systems.

Two-pipe systems are most common in residential areas. They supply heat for both space heating and domestic water heating via hot water (or steam). The cooled distribution medium returns to the central boiler via the return pipe. The heat supply must be at least 65°C to ensure safety for DHW from Legionella bacteria. A two-pipe system may also be used for space heating only; but this requires homeowners to have their own DHW heating systems.

Four-pipe systems, although more expensive, provide the greatest flexibility because space heating energy and DHW heating are served separately. As a result, the space heating supply circuit can operate at lower temperatures. Circulation losses are correspondingly smaller and the whole circuit can be switched off outside the heating season, which is a large part of the year for highperformance houses.

12.7.2 Designing district heating

Design of heat production by annual duration

Figure 12.7.1 illustrates a typical annual duration curve. The key design parameters are total system peak demand, average load requirements and load variations from day to day and season to season.

Many heating plants have a highly efficient or renewable energy system for the base load. This system typically covers 25 per cent to 40 per cent of the load and supplies 70 per cent to 90 per cent of the consumed heat on a yearly basis. Example plant types include:

• geothermal energy;

• solar collectors for space heating;

• combined heat and power generation (for example, fuel cells, motor engines, micro-turbines); and

• electrical or gas-driven heat pumps.

The peaks may be covered by a gas-fired boiler.

peak load

peak load o ci-

bas e load bas e load

Source: Carsten Petersdorff, Ecofys GmbH, Köln

2000

4000

6000

8000

10000

Figure 12.7.1 An annual duration curve

Hours

Peak shaving

To increase the share of the base load system, thermal storage is useful. The principle is to produce surplus heat and store it during low demand periods. As a result, the system can operate closer to its optimal efficiency throughout the day. When peak demand occurs, the stored heat can be drawn on. Because district heating systems service multiple users with varying peak load requirements, the load curve tends to be smoother than each individual load curve. The result is that both the heat supply and storage capacity can be proportionately lower than would be the case for heating plants and storage for each individual building.

Seasonal storage

Extrapolating the storage principle to a seasonal basis is very advantageous for solar use. With large seasonal storage systems, solar energy can be collected during summer and stored until needed in the heating season. Given the shortening of the heating season of high-performance houses to the sun-poor mid winter, this is a big advantage. The annual solar contribution can be as high as 70 per cent, even in a northern climate. Indeed, the first large solar heating plants were built during the late 1970s, with Sweden as the main pioneer. Other facilities followed in Denmark, The Netherlands and Finland, as well as Sweden. More recently, Germany has constructed several large demonstration systems.

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